This invention relates to papermaking, and more particularly to an apparatus and method for transporting a web along a nonlinear or curved path without allowing any machine components to come into contact with the web.
Coated paper or similar products are often manufactured in the form of a continuous web. The coating is applied to the web as a suspension in a solvent. The coated web is then passed through a dryer which removes the solvent leaving the desired dry coating on the surface of the web. The web can be paper, synthetic film or metallic foil, and the solvents used to apply the coating may be water or a wide variety of organic solvents or mixtures of solvents. Conventional heated cylinder dryers are usually unsuitable for the drying of such coated webs because the wet coating will stick to any contacted surface until it is substantially dry. For this reason, floater dryers have been developed and used to eliminate contact with a coated web prior to the drying of the coating on the web. The floater dryers float the web on a cushion of heated air, and these dryers dry the coating concurrently with providing non-contact support of the web as the web passes through the dryer.
At a coated web manufacturing installation, the floater dryers must be positioned with respect to the coater in a manner which enables the web to pass straight into the dryer from the coater without a change in direction which would require the coated side to contact a turning roll. Such a requirement severely restricts the arrangement of the various components, and the restrictions are even greater for machinery designed to coat simultaneously both sides of the web because there is no uncoated side that can be supported by a machine component. Ideally, a device which will support a moving web on a cushion of air as it makes the change of direction would solve many of these problems.
Apparatus which utilize an air cushion for floating and drying webs must insure the stability of the air flow with respect to the web to avoid harmful fluttering and related spurious movements of the web which can result in undesirable mechanical contact with the flow nozzles. In any turning device, the air cushion must also maintain sufficient pressure to react the components of the web tension which resist the turn.
Theoretical and experimental considerations show that floating a web on streams of air that flow parallel rather than perpendicular to the web results in orderly and stable web support while allowing a wide variety of nozzle-to-nozzle spacings to be utilized. Parallel air flow also provides for highly uniform heat transfer, and therefore drying, in both the cross machine and machine directions. U.S. Pat. No. 3,587,177 describes one type of nozzle which has been very successful in providing floatation drying utilizing a parallel flow of air. The device described in U.S. Pat. No. 3,587,177 creates a negative pressure which causes the web to run parallel to the nozzle face at a fixed distance on the order of 4-6 mm. Such nozzles can support a web from only one side or from both sides, and only along a straight line.
U.S. Pat. No. 4,414,757 provides another type of nozzle for the floatation drying of a coated web. This patent describes a device which creates a positive pressure cushion. For linear dryers, this nozzle must be used on both sides of a web in an alternate sequence in order to suitably react the positive pressure cushion and thereby control the movement of the web through the dryer.
It is therefore a principal object of the present invention to provide an apparatus for supporting a web for movement along a nonlinear path without contact between the apparatus and the web.
Another object of the present invention is to provide an apparatus for transporting and drying a coated web along a nonlinear path which does not require positioning machinery on both sides of the web.
Still another object of the present invention is to provide an apparatus for transporting and drying a coated web along a nonlinear path which can dry the coating on both sides of the web.